1. Field of the Invention
The present invention relates to a pneumatic vehicle tire comprising a carcass, a tread strip, a breaker assembly provided between the carcass and the tread strip and comprising at least two breaker plies reinforced by threads or cords of non-metallic material which extend at a predetermined angle to the mid-circumferential plane of the tire and serve as a reinforcing element, and a breaker cover or bandage which surrounds the breaker arrangement, and which comprises at least one cover ply having reinforcing thread or cords which extend at least substantially in the circumferential direction of the tire.
2. Description of the Background Art
Such pneumatic vehicle tires are customarily manufactured using a cylindrical drum. The individual tire components, such as the carcass, breaker plies, breaker cover and bandage and tread strip, and laid one after the other onto the drum. The tire carcass thus assembled is then shaped into a heated tire mold, in which the tire tread pattern is formed and the tire is vulcanized.
In order to shape the tire carcass into the heated tire mold, the individual tire components must be sufficiently radially extensible so that they can adopt the final arched form in the heated tire mold. On the other hand, a finished tire with a change of shape due to an increase of diameter of the tire under static or dynamic loading should be avoided. Breaker plies of a tire permit extension to a predetermined degree, as a result of their cords which are inclined at a predetermined angle to the mid-circumferential plane of the tire. Through a suitable choice of and material, this extension can only be achieved for the breaker cover or bandage which has cords extending substantially in the circumferential direction of the tire. In this respect, the breaker cover serves to prevent separation of the breaker components during the operation of the tire. Despite the extensibility necessary for the manufacture of the tire, the breaker cover must therefore have an adequate strength. In customer tires with steel breakers, polyamide it used as a material for the cords of the breaker cover. Nylon has the advantage that it has an adequate extensibility for the manufacturing process of the tire, but nevertheless provides the breaker cover the adequate strength during the operation of the tire. This is attributed to the fact that nylon shrinks during heating-up of the tire during rolling. As the tire heats up more and more with increasing rotational speed, the retaining force of the breaker cover increases by a corresponding amount. In fact in the vulcanization process the nylon shrinks due to the high temperature and thus generates a strong hoop tension in the compression in the steel breaker plies. Thus the nylon provides both a pre-tension effect and a secondary strengthening of the breaker assembly when rolling in service.
However, in tires with a non-metallic breaker arrangement, the problem now exists that the breaker plies only oppose the shrinkage force of a nylon bandage with a low resistance. The tire heated in the tire heating mold thus deforms as a result of the shrinkage force of a nylon breaker cover as soon as the pressure used for forming of the tire tread pattern is removed.
It has thus been proposed that a non-shrinking material should be used for the threads or cords of the breaker cover, which nevertheless ensures adequate retaining force even with rapidly running tires. A material of this kind is, for example, aramid, which is comparatively expensive. Moreover, when using aramid special measures must be taken in order to ensure adequate extensibility of the breaker cover during tire manufacture.